This invention relates to a novel process for the preparation of block copolymers modified by a maleic acid compound. More particularly, it relates to a novel process for preparing a thermoplastic polymer with excellent melt-flow characteristics, tensile strength, etc., by modifying a block copolymer composed of a conjugated diene compound and an aromatic vinyl compound with a maleic acid compound.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an aromatic vinyl compound by using an organic alkali metal initiator. These types of block copolymers are diversified in characteristics, ranging from rubber-like characteristics to resin-like characteristics, depending on the content of the aromatic vinyl compound.
When the content of the aromatic vinyl compound is small, the produced block copolymer is a so-called thermoplastic rubber. It is a very useful polymer which shows rubber elasticity in the unvulcanized state and is appliable for various uses such as moldings of shoe sole, etc.; impact modifier for polystyrene resins; adhesive; binder; etc.
The block copolymers with a high aromatic vinyl compound content, such as more than 70% by weight, provide a resin possessing both excellent impact resistance and transparency, and such a resin is widely used in the field of packaging. Many proposals have been made on process for the preparation of these types of block copolymers (Japanese Patent Publication Nos. 28915/72, 4106/73, etc.).
Also, many attempts have been made for the purpose of improving adhesiveness, green strength and other properties by modifying said resin or rubber with a maleic acid compound having high funactionality, and various methods have been proposed for modifying synthetic conjugated diene rubbers with a maleic acid compound (maleinizing). For instance, Japanese Patent Kokai (Laid-Open) No. 20294/74 shows a method for modifying polyisoprene in an inert solvent in the presence of a radical initiator. The main disadvantage of this method lies in complicated after-treatments such as solvent removal, and the like. Japanese Patent Publication No. 32707/70 discloses a method according to which polyisoprene is masticated while subjecting it to shearing force in a screw extruder without using any radical initiator and then maleic anhydride is added thereto to maleinize the polymer. This method is superior to the above-mentioned maleinization in an inert solvent in that the former is simpler in steps, but this method cannot be applied to maleinization of the block copolymers as in the present invention because in the case of a block copolymer in which the conjugated diene portion is mainly composed of butadiene, there takes place crosslinking, or gelation, in the butadiene portion during the mastication operation in which strong shearing force is given. On the other hand, in the case of a block copolymer in which the conjugated diene portion is mainly composed of isoprene, mastication may cause splitting of the molecular chain in the isoprene portion, resulting in a loss of structural characteristics as block copolymer. Japanese Patent Kokai (Laid-Open) No. 56427/75 suggests a method for maleinizing butadiene-styrene block copolymer in the presence of a rubber-extending oil using a radical initiator or generating a radical by heating. Japanese Patent Kokai (Laid-Open) No. 123443/77 suggests a method for maleinizing a butadiene-styrene block copolymer along with polyethylene in a screw extruder in the presence of a radical initiator. In any of these methods, however, maleinization is performed under generation of a radical, and therefore, gelation of the modified block copolymer obtained is unavoidable, and properties as thermoplastic polymer, such as melt-flow characteristics or appearance characteristics, of said modified block copolymer are lowered.